1. Field of the Invention
The present invention generally relates to semiconductor integrated circuits functioning as level-shifter circuits, and particularly relates to a semiconductor integrated circuit functioning as a level-shifter circuit that operates stably at high speed even with a low voltage input.
2. Description of the Related Art
Level-shifter circuits are used for the purpose of converting a signal having a predetermined voltage to a signal having a higher voltage level. A typical level-shifter circuit is disclosed in Japanese Patent Laid-open Application No. 6-37624, for example.
FIG. 1 is a circuit diagram showing a typical construction of a level-shifter circuit.
The level-shifter circuit of FIG. 1 includes PMOS transistors 11 and 12, NMOS transistors 13 and 14, and an inverter 15. An incoming signal IN is applied to the gate of the NMOS transistor 14, and is also inverted by the inverter 15 to be applied to the gate of the NMOS transistor 13. The incoming signal IN being HIGH (VL) makes the NMOS transistors 13 and 14 nonconductive and conductive, respectively, resulting in the PMOS transistors 11 and 12 being conductive and nonconductive, respectively. Accordingly, an output signal OUT is set to 0 V. If the incoming signal IN is LOW (0 V), the NMOS transistors 13 and 14 are conductive and nonconductive, respectively, resulting in the PMOS transistors 11 and 12 being nonconductive and conductive, respectively. In this case, therefore, the output signal OUT is set at VH. In this manner, the incoming potential level ranging between 0 and VL is shifted to a range between 0 and VH.
The transistors 11 through 14 that control the output signal OUT are designed for high-potential operations so as to properly operate at a potential range between 0 and VH. In general, high-speed signal transition requires the operating range of an incoming signal IN to be set at an increasingly lower potential level as the technology improves. In such a case, since the transistors 13 and 14 are designed for high-potential operations corresponding to the potential range boosted by the level shift, the potential level VL that is relatively low may not be able to turn on or off the transistors at sufficient speed. In some cases, the transistors may fail to be sufficiently conductive.
Accordingly, there is a need for a level-shifter circuit that can stably operate at high speed even when the incoming signal is set at a potential level significantly lower that the operating potential range of the circuit-component transistors.